Both Ge and Si crystallize in the diamond structure in space group Fd3m (aGe=5.6576 Å; aSi=5.4309 Å). The relatively small lattice mismatch (about 4.17%) makes it possible to grow highly ordered Ge on Si. Here highly ordered refers to the crystallographic alignments of the film along the x, y, and z directions and is very much determined by that of the crystal structure of the substrate as a result of some degree of lattice matching along the interface between the film and the substrate. Herein highly ordered germanium consists of aligned crystalline material in which the ratios of the peak intensities of (111)/(002) and the (202)/(002) Ge reflections in the θ-2θ XRD scan are <0.1 and the FWHM in the φ scan of the (202) reflection is <10°. Furthermore, the features of high carrier mobility and large absorption coefficient at near-infrared wavelengths make germanium (“Ge”) one of the most attractive semiconductor materials for a wide variety of applications. The small bandgap of Ge, for example, makes Ge a good candidate for photodetectors and modulators at wavelengths in the range of 1.3-1.6 micrometers (“μm”). The high carrier mobility of Ge makes it the choice for high-speed transistors that have potential applications in computers and switching systems.
Ge films have been prepared using molecular beam epitaxy (“MBE”) and chemical vapor deposition (“CVD”). These methods are relatively expensive, complex, and usually require a buffer layer in between the Ge layer and single crystal Si substrate to reduce lattice strain between the Ge layer and Si substrate. A simpler, less expensive method for preparing highly ordered Ge films on single crystal Si substrates is desirable.